Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Pulse width modulated (PWM) switching power supplies offer both compactness and efficiency in a number of different topologies which can be placed in two main categories: isolated switching power supplies and non-isolated switching power supplies. In a non-isolated switching power supply, such as a buck (reducing voltage) or boost (increasing voltage) switching power supply, the power output is not isolated from the power input. Isolated power supplies, such as a flyback or forward switching power supplies, have a power output that is isolated from the power input through a transformer.
In either type of power converter, however, typical control systems use a pulse-width-modulator to control the duty cycle of the power switch(es) within the converter. Consider, for example, the flyback switching power supply of FIG. 1. The power converter includes a power switch Q1 (typically a field effect transistor (FET)) coupled to the primary of a power transformer T1 and a diode D1 and capacitor C1 coupled to the secondary of the power transformer T1. The control system for controlling the power converter includes a PWM controller 105 to provide the signal to turn on switch Q1 and a feedback circuit 110 coupled to the PWM controller 105. The feedback circuit 110 receives an output power level sense circuit that varies in time with changes in the output power level. An oscillator (not shown) included in the PWM controller 105 sets the operating frequency while a pulse-width modulator adjusts the duty cycle of the power switch Q1 at the set operating frequency in response to sensing, for example, an output voltage, Vout. The frequency of the oscillator is relatively low, in the range of 50 KHz. The relationship between the input voltage, V.sub.in, and V.sub.out for the flyback converter illustrated in FIG. 1 may be approximated as EQU V.sub.O =(V.sub.IN *N.sub.S /N.sub.P)*D/(1-D); and EQU D=(T-toff)/T; and
N.sub.P --number of turns on the primary winding PA1 N.sub.S --number of turns on the secondary winding PA1 where `D` is duty cycle, T is the switching period, and t.sub.off is the off time of the power switch Q1.
Thus, in the flyback converter of FIG. 1, the off time, t.sub.off (and hence also the on time, t.sub.on) of the power switch Q1 defines a power cycle, or power pulse, which is reflected in the value of V.sub.out through the above equation. Similarly, the output voltage of a forward power converter can be determined using the equation: EQU V.sub.O =(V.sub.IN *N.sub.S /N.sub.P)* D
In any case, the power pulse is thus a regulated power pulse because its characteristics have a direct relationship on the output voltage. This relationship between the characteristics of a single power cycle (or pulse) and the output voltage is generic to prior art PWM switching power supplies, regardless of whether the PWM switching power supply is direct coupled or transformer coupled. Thus, a single power cycle (or pulse) in these prior art PWM switching power supplies may be denoted as an "intelligent" power cycle or pulse because of its effect on the output voltage.
FIG. 2 provides another example of a prior art PWM based power converter control system. In this case, the power converter is illustrated generally as the power stage 205 including a switching transistor Q1. The control system is indicated as controller 210 including the PWM controller 105 and feedback circuit 110. The feedback signal line is shown in this case to be a current sense on the output of power transistor Q1 and a connection to Vout of the power converter input to a summing circuit 215. As illustrated, this power converter PWM based control system may be used with any converter topology, whether isolated or non-isolated power converter configuration.
With this control system approach, the pulse widths of the pulses vary widely as input line voltage and output load conditions vary. Optimum system performance is achieved only at a single operating point (line and load condition), where the power pulse width and/or pulse frequency is well matched to the particular power conversion stage. Furthermore, because power pulses are closely coupled to output regulation, optimization over a wide operating range with only PWM control is difficult to achieve without degrading output regulation performance. Thus, there is a need in the art of power converters for a more versatile control system approach that can maintain optimal power converter performance and maintain high efficiency over a broad range of load and line conditions.